Process for preparing perfluoroalkyl bromides

ABSTRACT

The present invention provides a process for preparing a perfluoroalkyl bromide characterized by reacting a perfluoroalkyl iodide represented by the formula C n  F 2n+1  I wherein n is an integer of 6 to 10 with bromine in a light-transmitting reactor with exposure to light and heating at 120° to 180° C. while simultaneously removing IBr resulting as a by-product by separation of a layer.

This is a national stage application of PCT/JP93/01738 filed Nov. 30,1993.

TECHNICAL FIELD

The present invention relates to an efficient process for preparingperfluoroalkyl bromides. Perfluoroalkyl bromides are capable ofproviding a contrast in X-ray radiography and MR imaging, and arecompounds useful as base materials for such diagnostic chemicals.Advanced contrast media for X-ray radiography and MR imaging have beendeveloped especially from perfluorooctyl bromide.

BACKGROUND ART

To our knowledge, heat and light effect bromination perfluoroalkyliodides with bromine, whereas we have found that this reaction requiresa high temperature of at least 300° C. when to be effected thermallywith a high efficiency, or takes a long period of time when light isused for the progress of the reaction since IBr formed as a by-productblocks light.

Although the thermal reaction proceeds over a wide temperature range of40° to 500° C., the reaction must be conducted in the presence of aradical initiator (JP-A784033/1985) or at a high temperature of notlower than 300° C. (JP-A-287551/1991) if perfluoroalkyl bromides are tobe prepared efficiently. When the reaction is to be conducted at such ahigh temperature, however, the reactor and the piping therefor need tobe made of a material which is resistant to pressure and to corrosionwith bromine.

On the other hand, a report was made to the effect that a perfluoroalkylbromide was available when a perfluoroalkyl iodide and bromine wereirradiated, as enclosed in a glass tube, with ultraviolet rays for 7days R. N. Haszeldine, J. Chem. Soc., 3761-3768 (1953)!, whereas thereaction is on an experimental scale, requires a long period of time andtherefore appears industrially infeasible. Thus, preparation ofperfluoroalkyl bromides from perfluoroalkyl iodides and brominenecessitates a long period of time in the case of the photoreaction, ora radical initiator or high temperature in the case of the thermalreaction.

An object of the present invention is to provide a process forindustrially advantageously preparing a perfluoroalkyl bromide under amild heating condition efficiently within a short period of time and ina high yield without using a radical initiator or like additive.

DISCLOSURE OF THE INVENTION

The present invention provides a process for preparing a perfluoroalkylbromide characterized by reacting a perfluoroalkyl iodide represented bythe formula (n:6˜10) with bromine in a light-transmitting reactor withexposure to light and heating at 120° to 180° C. while simultaneouslyremoving IBr resulting as a by-product by separation of a layer.

The perfluoroalkyl iodide represented by the above formula and to beused as the starting material in the present invention may be astraight-chain or branched compound. Examples of such iodides are CF₃(CF₂)₅ I, (CF₃)₂ CF(CF₂)₄ I, CF₃ (CF₂)₅ I, CF₃ (CF₂)₇ I and the like. Inthe above formula, n is more preferably in the range of 6 to 8.Preferably, the ratio between the reactants is, for example, about 1 toabout 3 moles of bromine per mole of the perfluoroalkyl iodide.

Although the reaction is conducted by placing the starting material,i.e., perfluoroalkyl iodide, into a light-transmitting reactor andadding bromine dropwise to the iodide with heating and exposure tolight, a small amount of bromine may be added to the material before thereaction. The reaction can be carried out also in a continuous mode.FIG. 2 is a flow chart showing the continuous mode. Usable as thelight-transmitting reactor is, for example, a flask or tube made ofglass or other reactor made of glass. The reaction temperature ispreferably 120° to 180° C. at which the perfluoroalkyl iodide generallyrefluxes, more preferably 140° to 160° C. If lower than theabove-mentioned lower limit, the reaction temperature is lower than theboiling point of IBr at atmospheric pressure, making it impossible toremove IBr and consequently resulting in a greatly reduced reactionvelocity. If the temperature is higher than the upper limit, thereaction system becomes difficult to handle owing to vigorous reflux.Usable as the light are visible rays, and rays, such as ultravioletrays, which are up to 1 μm, preferably 0.2 to 0.7 μm, in wavelength.Furthermore, decomposition of the material is avoidable effectively byconducting the reaction in a nitrogen stream.

The IBr formed as a by-product by the present process is approximate tothe starting material, perfluoroalkyl iodide, and the desired product,perfluoroalkyl bromide, in boiling point, and was difficult to separatefrom the starting material and the desired product by distillation only.However, we have found that when the mixture of IBr, starting materialand desired product is allowed to stand, the IBr separates in the formof a lower layer since IBr is sparingly soluble in the starting materialand the desired product and greater than the material and the product inspecific gravity. Accordingly, when the reactor has a reflux tubeprovides at a lower portion thereof with a trap for separating a layer,the IBr separates in the form of a lower layer and can be withdrawncontinuously during the reaction by drawing off the IBr from a lowerportion of the trap. The withdrawal of light-blocking IBr from thereaction system in the form of a separate layer permits the light toachieve an improved reaction efficiency, and is advantageous also inincreasing the concentration of the starting material.

As described above, the present invention attains an improved reactionefficiency and affords the desired product in a high yield within ashort period of time by continuously withdrawing from the system theby-product of IBr as separated in the form of a layer. After thecompletion of the reaction, the desired product, i.e., a perfluoroalkylbromide, can be obtained readily, for example, by washing the reactionmixture with an alkali or reducing agent and subsequently rectifying themixture.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an example of a reactor adapted for continuous withdrawalof IBr for use in the invention;

FIG. 2 is an example of flow chart of a continuous reaction for use inthe invention; and

FIG. 3 is a graph showing the yields of C₈ F₁₇ Br with time achieved inExamples 1 and 2, and Comparative Examples 1 to 3.

BEST MODE OF CARRYING OUT THE INVENTION

The present invention will be described with reference to the followingexamples.

EXAMPLE 1

With reference to FIG. 1, and H-shaped tube equipped with a Dimrothcondenser and a Dimroth condenser having an IBr withdrawing cock wasattached to a 500-ml four-necked flask. Into the flask were placed 200ml (747 mM) of perfluorooctyl iodide (PFOI) and 5.0 ml (97.1 mM) ofbomine, and the mixture was heated at 150° to 155° C. and irradiatedwith a 500-W visible light lamp. After the reaction mixture started toreflux, 49.5 ml (972 mM) of bromine was added dropwise to the mixture ata rate of 0.2 ml/min over a period of about 4 hours. The resulting IBrwas drawn off at a suitable time through the IBr withdrawing Dimrothcondenser. The reaction was continued for 5 hours, and the reactionmixture was thereafter cooled and washed with a 5% aqueous solution (200ml) of sodium hydroxide to obtain 344.9 g (92.5% in yield) ofperfluorooctyl bromide.

EXAMPLE 2

The same reactor as in Example 1 was used. Into the flask was placed 200ml (747 mM) of perfluorooctyl iodide (PFOI), which was then heated at160° C. and irradiated with a 400-W high-pressure mercury lamp. Afterthe reaction mixture started to reflux, 54.5 ml (1.07M) of bromine wasadded dropwise to the mixture at a rate of 0.4 ml/min over a period ofabout 2 hours. The resulting IBr was drawn off at a suitable timethrough the IBr withdrawing Dimroth condenser. The reaction wascontinued for 3 hours, and the reaction mixture was thereafter cooledand washed with a 5% aqueous solution (200 ml) of sodium hydroxide,giving 349.0 g (93.6% in yield) of perfluorooctyl bromide.

Comparative Example 1

A reaction was conducted in the same manner as in Example 1 except thatno IBr was withdrawn, consequently giving perfluorooctyl bromide (C₈ F₁₇Br) in a yield of 12%.

Comparative Example 2

A reaction was conducted in the same manner as in Example 2 except thatno IBr was withdrawn, consequently giving perfluorooctyl bromide (C₈ F₁₇Br) in a yield of 19%.

Comparative Example 3

A reaction was conducted in the same manner as in Example 1 with theexception of withdrawing no IBr and using not light for irradiation,consequently giving perfluorooctyl bromide (C₈ F₁₇ Br) in a yield of 4%.

Comparative Example 4

A reaction was conducted in the same manner as in Example 1 except thatthe reaction temperature was 110° C., consequently giving perfluorooctylbromide (C₈ F₁₇ Br) in a yield of 5M.

FIG. 3 shows the yields of C₈ F₁₇ Br achieved with time in Examples 1and 2, and Comparative Examples 1 to 3.

INDUSTRIAL APPLICABILITY

The invention makes it possible to industrially advantageously prepare aperfluoroalkyl bromide efficiently within a short period of time and ina high yield under such a mild heating condition as to permit reflux ofa perfluoroalkyl iodide without using any additive.

We claim:
 1. A process for preparing a perfluoroalkyl bromide comprisingreacting a perfluoroalkyl iodide represented by the formula C_(n)F_(2n+1) I wherein n is an integer of 6 to 10 with bromine in alight-transmitting reactor with exposure to light and heating at 120° to180° C. while simultaneously removing IBr resulting as a by-product byseparation of a layer.
 2. A process for preparing a perfluoroalkylbromide as defined in claim 1 wherein the reactor has a reflux tubeprovided at a lower portion thereof with a trap for separating saidlayer, and said IBr separating as a lower layer in the trap is removed.3. A process for preparing a perfluoroalkyl bromide as defined in claim1 wherein the perfluoroalkyl iodide is represented by the formula C_(n)F_(2n+1) I wherein n is an integer of 6 to
 8. 4. A process for preparinga perfluoroalkyl bromide as defined in claim 1 wherein about 1 to about3 moles of the bromine is reacted with 1 mole of the perfluoroalkyliodide.
 5. A process for preparing a perfluoroalkyl bromide as definedin claim 1 wherein said heating is conducted at a temperature in therange of from 140° to 160° C.